The present inventions relate to the field of thin film devices and processes. More particularly, the present inventions relate to thin film solar cells and thin film flexible circuit boards, and methods of manufacturing the same.
There has been recently interest in the conceptual designs for microsatellites and nanosatellites for various space missions deployed, for example, in low earth orbits. These microsatellites and nanosatellites require the collection of sufficient power for onboard instruments. These microsatellites and nanosatellites are typically low in weight and low in volume and have a limited amount of surface area for power collection. Because the overall surface area of a microsatellite or nanosatellite is small, body-mounted solar cells may not provide enough power for the on-board instruments. Traditional rigid solar arrays necessitate larger satellite volumes and weights and also require extra apparatus for pointing the spacecraft solar arrays.
The limited surface area of the microsatellites or nanosatellites is the power choke problem where insufficient energy is collected. One potential solution to the power choke problem is the use of a spherical deployable power system having a spherical outer surface covered with solar cells offering a high collection with low weight and low stowage volume, while eliminating the need for a solar array pointing mechanism. For powering spacecraft, the collection of solar energy requires the exposure of solar cells to sunlight. The thin film solar cell must be capable of maintaining thermal equilibrium by radiating all thermal energy absorbed while operating in a space vacuum environment. Solar cells absorb nearly 90% of the incident sunlight and convert a small portion, for example 5-20%, of that energy to electricity. To maintain thermal equilibrium, the solar array has thermal properties that allow rejection of the total solar input as thermal radiation. The solar array design uses materials that have the appropriate thermal radiation coefficients to allow thermal equilibrium to be reached at a temperature that is optimum for the efficient operation of the solar cells.
The power sphere is a curved electrical power system. However, modern solar cell panels are typically fabricated using a plurality of rigid solar cell panels unsuitable for flexible forming about curved surfaces. For thermal radiation of heat absorbed from incident solar radiation and for the collection of energy through solar illumination, typically a rigid solar cell would be bonded using adhesives to a thick transparent cover glass. This same basic fabrication methodology has been transferred to the production of thin film solar arrays by using an adhesive to bond a transparent polymer on the top of the thin film solar cell. The adhesive is subject to damage and failure through thermal cycling, radiation and solar ultra violet illumination.
Thin film solar cells have been deposited on kapton polyimide forming an integral flexible thin film solar cell that is then bonded between opposing sheets of polymer (i.e. Tefzel) having the required transparent and thermal emissive properties. These thin film solar cells have been used in terrestrial application but suffer from the use of bonding adhesive completely covering the surfaces of the sheet polymer tefzel used for terrestrial environmental protection.
Thin film solar cells could be bonded using the adhesive to flexible thin film circuit boards. Traditional flexible printed circuit boards are fabricated by laying copper circuitry down on both sides of a flexible substrate and then using adhesive to bond subsequent pairs of circuits or alternating a polymer material layer with the copper circuitry and laminating each pair of layers to bond the pairs. The multiple layers of polymer material bonded together with an adhesive with each layer having a different coefficient of thermal expansion would induce differential mechanical stress between the various layers at different operating temperatures. This arrangement is unsuitable for flexible thin film solar arrays because the adhesive layer may be damaged by temperatures required for deposition of the thin film solar cell on the flexible circuit, because the adhesive layer adds weight and might result in relatively thick films, and because the multiple layers of polymer material film and adhesive would result in a film with differences in coefficients of thermal expansion that would induce different mechanical stresses in the composite material at different temperatures during solar illumination cycles. The multiple layers of polymer material and adhesive layers significantly increases the mass and complexity of the power sphere. For commercial terrestrial uses, this problem of alternate layers of adhesive and polymer might be manageable, but for the space environment this delaminating problem is much greater due to the wide temperature extremes that the power sphere will be exposed to during different parts of each orbit. In addition, the total number of thermal cycles that a low earth orbiting satellite experiences on a daily basis is far greater than one would expect for most terrestrial applications.
The basic architecture of the power management and control system for the power sphere requires regulation of each individual solar cell mounted on the surface of the sphere. This power management and control system has not been integrated with flexible thin film solar cells suitable for flexible forming about a small curved surface such as the exterior of a nanosatellite or microsatellite. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a flexible space qualified thin film solar cell having a thermal emissive layer for heat rejection.
Another object of the invention is to provide a flexible circuit board having multiple layers of deposited thin films adhered together without the use of bonding adhesives.
Yet another object of the invention is to provide a flexible solar cell power module having a top thermal emissive layer and a bottom flexible circuit board integrally fabricated using thin film processes.
The present inventions are directed to a flexible thin film circuit, such as thin film solar cells, and methods of manufacturing the same. In order to reduce mass of the microsatellite, an integrated thin film solar power module is used. The thin film solar cell with associated interconnects and power processing electronics for regulating the solar generated electricity are integrated into the polymer material substrate on which the thin film solar cell is fabricated. The thin film solar cell is fabricated on a flexible printed circuit board. The flexible printed circuit board is made of a polymer material with the required copper traces for the electronic circuit embedded within the thickness of the polymer film.
In order to simplify the wiring harness of a power sphere, a preferred location for the required power regulation electronics required for each solar cell is on the backside of the substrate for the solar cell. The circuit traces for the power processing electronics are not bonded to flexible thin film solar cells, but are integrated into thin film layer by deposition of circuit traces within thin films forming a thin film circuit board, which is also the substrate for the thin film solar cell. The integrated thin film assembly can be used in the power sphere with reduced mass and wiring. The flexible thin film solar power module is connected to a simple two wire bus eliminating the need to run individual wires from each solar cell to a central grouping of power processing electronics.
The integrated multiple layer flexible thin films in an integrated circuit board eliminates the need for multiple adhesive layers between multiple polymer layers thereby eliminating large variants in thermal coefficient of expansion and difference in thermal emissivity. The fabrication of multiple thin films upon one another without the use of adhesives enables the manufacture of a flex circuit board that has only two components, which are the polymer material and the embedded metal for the circuit traces. This improved flex circuit board can be fabricated with extremely thin layers of polymer material on which the metal circuit traces are deposited between deposition of subsequent layers of polymer material. The resultant flexible circuit board can be utilized as the substrate for depositing the thin film solar cell. Another top thin film layer of clear polymer can be deposited on top of the solar cell thereby providing proper thermal emissivity without the use of an adhesive. Hence, the solar power module comprising a top thermal emissive layer, center thin film solar cell and the bottom flex circuit is an integrated flexible thin film solar cell power module. Additionally, the integrated flexible solar cell power module could be further enhanced with mounted discrete power electronic components on the backside of the flexible circuit board assembly for providing necessary electronic processing control. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.